1. Field of the Invention
This invention relates generally to a fuel cell based system employing a load following algorithm and, more particularly, to a fuel cell based system that employs a load following algorithm, where the system includes a current sensor for measuring the current drawn from the fuel cell, and where the load following algorithm selectively controls the fuel and air applied to the fuel cell so that a predetermined amount of extra fuel and air is applied to the fuel cell to satisfy sudden increases in power demand.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines. Hydrogen fuel cells can also be a clean and efficient energy supply for stationary power, referred to as distributed generation.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas as a fuel and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode where they react with the oxygen and the electrons in the cathode to generate water. The electrons disassociated in the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a known popular fuel cell. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and the cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Typically, a system employing a fuel cell for generating power includes a fuel cell distributed generation (FCDG) system that generates a conditioned direct current (DC) or alternating current (AC) to provide the desired power requirements for a particular application. The FCDG system provides the amount of power based on the demand from the system loads at a particular point in time. For an automotive application, the vehicle operator presses the power pedal to generate more vehicle speed, which requires more output power from the fuel cell. The power request is made to the powertrain of the vehicle. The additional power to increase the vehicle speed is not provided to the powertrain until it is being produced and is available from the fuel cell. Thus, it takes a certain amount of time from the time that the operator presses the power pedal until the desired amount of power is provided by the fuel cell. Sometimes this time period is on the order of 500 milliseconds.
In other applications, such as home power generation applications, the FCDG system will draw all the power it needs immediately from the fuel cell, possibly drawing more power than the fuel cell is able to provide from its current fuel and air input. For example, a user may flip a switch to start an appliance where the added power demand is necessary almost instantaneously. When this happens, the power draw from the fuel cell could damage the fuel cell by attempting to draw more current than the fuel cell is capable of delivering at that moment in time. Thus, known FCDG systems employ an additional power source, such as a battery, in parallel with the fuel cell to meet the additional power requirements during the transient time between when the power request is made and the fuel cell begins generating the additional power.
An FCDG system can employ a load following algorithm that conditions and provides the desired amount of output power virtually instantaneously to satisfy the loads as they are connected and disconnected to and from the FCDG system. To do this, the load following algorithm manages the dual power sources of the battery and the fuel cell to reject and control the disturbances imposed on the fuel cell by the changing loads.
FIG. 1 is a general schematic block diagram of an FCDG system 10. The system 10 includes a fuel cell 12 that generates output power based on the principles discussed above. The system 10 also includes a storage battery 14 that provides additional power during start-up and at those times that the fuel cell 12 is not providing enough power to operate certain distributed generation (DG) loads 24. The system 10 also includes a power conditioning module 18 that includes DC/DC converters and DC/AC inverters for converting the DC power from the fuel cell 12 to DC power of various voltage levels and the DC power from the fuel cell 12 to AC power for the loads 24.
The fuel cell 12 provides variable DC power on output line 16 to the power conditioning module 18 depending on the fuel and air input to the fuel cell 12. Likewise, the battery 14 provides DC power on line 20 to the power conditioning module 18, such as for example, 60 kilowatts DC power. The power conditioning module 18 includes the appropriate circuitry to condition the DC power to different DC power levels and to AC power. The AC power is provided on line 22 to operate the various DG loads 24 depending on the particular application. The DG loads 24 can switch on and off at any time, resulting in less or more power draw from the fuel cell 12.
The power conditioning module 18 provides conditioned DC power on line 26 for certain devices in the fuel cell 12, such as 380 volts for a system compressor that provides the cathode input air. The power conditioning module 18 also provides conditioned DC power on line 28 to the fuel cell 12 at a lower voltage level than the line 26, such as 12 and/or 42 volts, to operate other fuel cell components, such as low power ancillary components. The power conditioning module 18 also provides DC power on line 30 to charge the battery 14 during those times that the fuel cell 12 is providing more power than is required by the DG loads 24.
Certain constraints are imposed on the control system operating the system 10. Particularly, current drawn on the line 16 from the fuel cell 12 (I_fuelcell) should not exceed the current available from the fuel cell stack (I_maxFC). Further, the rate of change of the flow of current from the fuel cell 12 is limited as a result of gas flow dynamics. Testing, durability concerns and system components collectively define the flow dynamics. For this disclosure, the flow dynamic is limited to:                                           ⅆ                          (              I_fuelcell              )                                            ⅆ            t                          ≤                  25          ⁢                                           ⁢                      (                          amps/s                        )                                              (        1        )            Additionally, the battery output voltage should be maintained between 62 V and 70 V. Battery current during charging has to be limited to avoid battery boil off, i.e., I_batt≧10 amps into the battery. Also, the fuel cell voltage has to be maintained within a certain percentage of its polarization curve. The fuel cell diagnostics would shut the system down if these parameters were violated to protect the fuel cell from irreversible damage.
As discussed above, FCDG systems can employ load following or load balancing algorithms to power balance the primary load, the compressor load and the low power ancillary loads with the power generator by the fuel cell 12. If the power on the lines 16, 20, 22, 26, 28 and 30 can be accurately measured by suitable sensors and the efficiency of the power conditioning module 18 is accurately known, then the power balance can be expressed as:                     η        =                                            P              cmp                        +                          P              anc                        +                          P              load                                                          P              fuelcell                        +                          P              batt                                                          (        2        )            In equation (2), Pcmp is the compressor power on the line 26, Panc is the ancillary power on the line 28, Pload is the AC load on the line 22, Pfuelcell is the power provided by the fuel cell 12 on the line 16, Pbatt is the power on the line 20 from the battery 14, and η is the overall efficiency of the power conditioning module 18.
Thus, if the efficiency η is known and all the power is measured in the FCDG system by current and voltage sensors, then Pfuelcell can be calculated. However, if any of the power measurements are underestimated, the power requirement can cause the battery 14 to be drained over a period of time. On the other hand, if the sensors over-estimate the power measurements, the system 10 will operate inefficiently. In addition to being able to accurately measure the currents discussed herein, such a scheme would require at least ten voltage and current sensors, and a fully developed efficiency map over the entire operating range of the system 10. Thus, such a technique may be very fault intolerant, costly and inefficient.
Fuel cell stoichiometry or lambda defines the amount of fuel and air that is necessary to generate a particular output current from the fuel cell. Particularly, the fuel lambda, also called anode stoichiometry, is the moles per second of hydrogen delivered to the fuel cell stack divided by the moles per second of hydrogen consumed by the fuel cell stack. Likewise, the oxygen lambda is the moles per second of oxygen delivered to the fuel cell stack divided by the moles per second of oxygen consumed by the stack. If the fuel cell stack operated perfectly, then the fuel lambda and the oxygen lambda would be one. However, because the fuel and oxygen do not reach all of the catalyst evenly and perfectly in the stack, more fuel and oxygen is required to provide a particular output current, and thus the fuel lambda and the oxygen lambda are necessarily greater than one.